An MR sensor detects magnetic field signals through the resistance changes of a read element, fabricated of a magnetic material, as a function of the strength and direction of magnetic flux being sensed by the read element. The conventional MR sensor, such as that used in the IBM "Corsair" disk drive, operates on the basis of the anisotropic magnetoresistive (AMR) effect in which a component of the read element resistance varies as the square of the cosine of the angle between the magnetization in the read element and the direction of sense current flow through the read element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the read element, which in turn causes a change in resistance in the read element and a corresponding change in the sensed current or voltage.
A different and more pronounced magnetoresistance, called giant magnetoresistance (GMR) or spin valve magnetoresistance (SVMR), has been observed in a variety of magnetic multilayered structures, the essential feature being at least two ferromagnetic metal layers separated by a nonferromagnetic metal layer. This GMR effect has been found in a variety of systems, such as Fe/Cr or Co/Cu multilayers exhibiting strong antiferromagnetic coupling of the ferromagnetic layers, as well as in essentially uncoupled layered structures in which the magnetization orientation in one of the two ferromagnetic layers is fixed or pinned. The physical origin is the same in all types of structures: the application of an external magnetic field causes a variation in the relative orientation of the magnetizations of neighboring ferromagnetic layers. This in turn causes a change in the spin-dependent scattering of conduction electrons and thus the electrical resistance of the structure. The resistance of the structure thus changes as the relative alignment of the magnetizations of the ferromagnetic layers changes.
A particularly useful application of GMR is a sandwich structure comprising two essentially uncoupled ferromagnetic layers separated by a nonmagnetic metallic spacer layer in which the magnetization of one of the ferromagnetic layers is "pinned". The pinning may be achieved by depositing onto the ferromagnetic layer to be pinned an antiferromagnetic iron-manganese (Fe--Mn) layer so that these two adjacent layers are exchange coupled. The unpinned or "free" ferromagnetic layer has the magnetization of its extensions (those portions of the free layer on either side of the central sensing region) also fixed, but in a direction perpendicular to the magnetization of the pinned layer so that only the magnetization of the free layer central region is free to rotate in the presence of an external field. Typically, the magnetization in the free layer extensions is also fixed by exchange coupling to an antiferromagnetic layer. However, the antiferromagnetic material used for this must be different from the Fe--Mn antiferromagnetic material used to pin the pinned layer. The resulting structure is a spin valve magnetoresistive (SVMR) sensor in which only the free ferromagnetic layer is free to rotate in the presence of an external magnetic field. U.S. Pat. No. 5,206,590, assigned to IBM, discloses a basic SVMR sensor. U.S. Pat. No. 5,159,5 13, also assigned to IBM, discloses a SVMR sensor in which at least one of the ferromagnetic layers is of cobalt or a cobalt alloy, and in which the magnetizations of the two ferromagnetic layers are maintained substantially perpendicular to each other at zero externally applied magnetic field by exchange coupling of the pinned ferromagnetic layer to an antiferromagnetic layer.
The SVMR sensor that has the most linear response and the widest dynamic range is one in which the magnetization of the pinned ferromagnetic layer is parallel to the signal field and the magnetization of the free ferromagnetic layer is perpendicular to the signal field. In the case where the SVMR sensor is to be used in a horizontal magnetic recording disk drive, this means that the plane of the sensor is perpendicular to the disk surface with the magnetization of the pinned layer oriented perpendicular to, and the magnetization of the free layer oriented parallel to, the disk surface. One difficulty in achieving this magnetization orientation is caused by the dipole field generated by the pinned layer. The pinned layer has a net magnetic moment and thus essentially acts as a macroscopic dipole magnet whose field acts on the free layer. In SVMR sensors where the height of the read element is relatively small, the result of this magnetostatic coupling is that the magnetization direction in the free layer is not uniform. This causes portions of the sensor to saturate prematurely in the presence of the signal field, which limits the sensor's dynamic range and thus the recording density and overall performance of the magnetic recording system.
The related copending '477 application relates to a SVMR sensor that addresses this problem by the use of a multiple film, laminated, pinned ferromagnetic layer in place of the conventional single-layer pinned layer. The laminated pinned layer has at least two ferromagnetic films antiferromagnetically coupled to one another across a thin antiferromagnetically (AF) coupling film. Since the pinned ferromagnetic films have their magnetic moments aligned antiparallel with one another, the two moments can be made to essentially cancel one another. As a result, there is essentially no dipole field to adversely affect the free ferromagnetic layer.
In SVMR sensors that use either the single-layer pinned layer or the laminated pinned layer described in the copending application, the preferred method of pinning the layer is by exchange coupling with an Fe--Mn antiferromagnetic layer. The use of Fe--Mn as the exchange coupling layer presents several problems. The exchange field strength generated by the Fe--Mn is highly sensitive to temperature. As the temperature increases, the Fe--Mn "softens" and its ability to fix the magnetization of the pinned ferromagnetic layer decreases. Thus, SVMR sensors can be damaged by electrostatic discharge (ESD) current and the resultant heating of the Fe--Mn. Fe--Mn is also much more susceptible to corrosion than the other materials used in the SVMR sensor. This fact requires careful control of the fabrication process steps and the use of protective materials for the SVMR. The use of Fe--Mn also requires that the antiferromagnetic material used to exchange bias the extensions of the free ferromagnetic layer be made of a different material, preferably Ni--Mn. To provide sufficient exchange coupling field strength, the Ni--Mn must be annealed at approximately 240.degree. C. At this temperature, interdiffusion of the other materials into the free ferromagnetic layer can occur. This can result in decreased magnetoresistance, increased anisotropy field strength, and a large change in magnetostriction of the free ferromagnetic layer.
What is needed is a SVMR sensor that has none of the disadvantages associated with an Fe--Mn exchange coupling layer, and has a pinned ferromagnetic layer that causes minimal magnetostatic coupling with the free ferromagnetic layer.